Bounce messages look impenetrable and contain the answer.
THE STRUCTURE
A summary saying delivery failed The recipient address that failed A status code and an explanatory line The original message headers Sometimes the original message
WHAT TO LOOK FOR
The recipient address: confirm it is spelled correctly
The code: 4xx is temporary, 5xx is permanent
The explanatory text, which is the part that matters The name of the server that rejected it, which tells you whether the refusal came from our side or theirs
WHO REJECTED IT
If the rejecting server is ours, the problem is at your end: authentication, sending limits, or a full mailbox.
If the rejecting server belongs to the recipient's provider, the problem is their policy or your reputation.
THE PRACTICAL POINT
Do not delete bounce messages before reading them. They are the only direct evidence of what happened, and forwarding one to us in a ticket is far more useful than describing it.
